Analysis of Flexural Fatigue Damage in Unidirectional Composites

Abstract

A flexural fatigue study of unidirectional E-glass/epoxy composite with 48 volume % glass fibers was undertaken (1) to understand and analyze the mechanisms of fatigue damage and (2) to evaluate the material degradation due to cyclic loading by monitoring the loss in stiffness. The fatigue tests were performed in the deflection-controlled mode with a sinusoidal waveform for a frequency of 3 Hz. The deflection-controlled testing mode was chosen to allow for decreased tendency toward hysteretic heating and to maintain controlled damage growth during fatigue cycling. Also, this testing mode minimizes the creep-fatigue interaction. Fatigue failure mechanisms in high and low deflection levels were found to be different. At high deflection levels, both matrix cracks and fiber breaks preferentially oriented at 90° to the fiber direction occurred simultaneously on the tensile side of the specimen, and out-of-plane fiber buckling was evident on the compression side, which eventually resulted in localized delamination. At low deflection levels, matrix cracking and fiber-matrix debonding were evident on the tensile surface. Fiber breakage was either not present or was very minimal. Compressive damage was not a concern at low deflection levels. The stiffness loss at high deflection cycling was rapid as opposed to a gradual decrease observed at low deflection cycling. The greater stiffness loss at the higher deflection levels was attributed primarily to glass fiber breaks.